The recent increase in the use of portable electronic devices such as mobile telephones and notebook computers and the emerging trend of using rechargeable batteries in hybrid electric vehicles has created a need for smaller, lighter, longer lasting rechargeable batteries to provide the power to the above mentioned and other battery powered devices. During the 1990s, lithium rechargeable batteries, specifically lithium-ion batteries, became popular and, in terms of units sold, now dominate the portable electronics marketplace and are set to be applied to new, cost sensitive applications. However, as more and more power hungry functions are added to the above mentioned devices (e.g. cameras on mobile phones), improved and lower cost batteries that store more energy per unit mass and per unit volume are required.
The basic composition of a conventional lithium-ion rechargeable battery cell including a graphite-based anode electrode is shown in FIG. 1. The battery cell includes a single cell but may also include more than one cell.
The battery cell generally comprises a copper current collector for the anode 10 and an aluminium current collector for the cathode 12 which are externally connectable to a load or to a recharging source as appropriate. A graphite-based composite anode layer 14 overlays the current collector 10 and a lithium containing metal oxide-based composite cathode layer 16 overlays the current collector 12. A porous plastic spacer or separator 20 is provided between the graphite-based composite anode layer 14 and the lithium containing metal oxide-based composite cathode layer 16; a liquid electrolyte material is dispersed within the porous plastic spacer or separator 20, the composite anode layer 14 and the composite cathode layer 16. In some cases, the porous plastic spacer or separator 20 may be replaced by a polymer electrolyte material and in such cases the polymer electrolyte material is present within both the composite anode layer 14 and the composite cathode layer 16.
When the battery cell is fully charged, lithium has been transported from the lithium containing metal oxide via the electrolyte into the graphite-based layer where it reacts with the graphite to create the compound, LiC6. The graphite, being the electrochemically active material in the composite anode layer, has a maximum capacity of 372 mAh/g. It will be noted that the terms “anode” and “cathode” are used in the sense that the battery is placed across a load.
It is well known that silicon can be used as the active anode material of a rechargeable lithium-ion electrochemical battery cell (see, for example, Insertion Electrode Materials for Rechargeable Lithium Batteries, M. Winter, J. O. Besenhard, M. E. Spahr, and P. Novak in Adv. Mater. 1998, 10, No. 10). It is generally believed that silicon, when used as an active anode material in a lithium-ion rechargeable cell, can provide a significantly higher capacity than the currently used graphite. Silicon, when converted to the compound Li21Si5 by reaction with lithium in an electrochemical cell, has a maximum capacity of 4,200 mAh/g, considerably higher than the maximum capacity for graphite. Thus, if graphite can be replaced by silicon in a lithium rechargeable battery the desired increase in stored energy per unit mass and per unit volume can be achieved.
Many existing approaches of using a silicon or silicon-based active anode material in a lithium-ion electrochemical cell, however, have failed to show sustained capacity over the required number of charge/discharge cycles and are thus not commercially viable.
One approach disclosed in the art uses silicon in the form of a powder having particles with a diameter of 10 μm in some instances made into a composite with or without an electronic additive and containing an appropriate binder such as polyvinylidene difluoride; this anode material is coated onto a copper current collector. However, this electrode system fails to show sustained capacity when subjected to repeated charge/discharge cycles. It is believed that this capacity loss is due to partial mechanical isolation of the silicon powder mass arising from the volumetric expansion/contraction associated with lithium insertion/extraction to and from the host silicon. In turn this gives rise to electrical isolation of the silicon particles from both the copper current collector and each other. In addition, the volumetric expansion/contraction causes the individual particles to be broken up causing a loss of electrical contact within the spherical element itself.
Another approach known in the art designed to deal with the problem of the large volume changes during successive cycles is to make the size of the silicon particles that make up the silicon powder very small, i.e. in the 1-10 nm range. This strategy does not prevent the electrical isolation of the spherical elements from both the copper current collector and themselves as the silicon powder undergoes the volumetric expansion/contraction associated with lithium insertion/extraction. Importantly, the large surface area of the nano-sized elements can give rise to the creation of a lithium-containing surface film that introduces a large irreversible capacity into the lithium-ion battery cell. In addition, the large number of small silicon particles creates a large number of particle-to-particle contacts for a given mass of silicon and these each have a contact resistance and may thus cause the electrical resistance of the silicon mass to be too high.
The above problems have thus prevented silicon particles from becoming a commercially viable replacement for graphite in lithium rechargeable batteries and specifically lithium-ion batteries.
In another approach described by Ohara et al. in Journal of Power Sources 136 (2004) 303-306 silicon is evaporated onto a nickel foil current collector as a thin film and this structure is then used to form the anode of a lithium-ion cell. However, although this approach gives good capacity retention, this is only the case for very thin films (say ˜50 nm) and thus these electrode structures do not give usable amounts of capacity per unit area.
A review of nano- and bulk-silicon-based insertion anodes for lithium-ion secondary cells has been provided by Kasavajjula et al (J. Power Sources (2006), doi:10.1016/jpowsour.2006.09.84), herewith incorporated by reference herein.
Another approach described in UK Patent Application GB2395059A uses a silicon electrode comprising a regular or irregular array of silicon pillars fabricated on a silicon substrate. These structured silicon electrodes show good capacity retention when subjected to repeated charge/discharge cycles and this good capacity retention is considered by the present inventors to be due to the ability of the silicon pillars to absorb the volumetric expansion/contraction associated with lithium insertion/extraction from the host silicon without the pillars being broken up or destroyed. However, the structured silicon electrodes described in the above publication are fabricated by using a high purity, single crystal silicon wafer and hence the electrode is expensive.
Selective etching of silicon-based materials to create such silicon pillars is known from U.S. Pat. No. 7,033,936. According to this document, pillars are fabricated by creating a mask by depositing hemispherical islands of caesium chloride on a silicon substrate surface, covering the substrate surface, including the islands, with a film, and removing the hemispherical structures (including the film covering them) from the surface to form a mask with exposed areas where the hemispheres had been. The substrate is then etched in the exposed areas using reactive ion etching and the resist is removed, e.g. by physical sputtering, to leave an array of silicon pillars in the unetched regions, i.e. in the regions between the locations of the hemispheres.
An alternative, chemical approach is described in Peng K-Q, Yan, Y-J, Gao S-P, and Zhu J., Adv. Materials, 14 (2002), 1164-1167, Adv. Functional Materials, (2003), 13, No 2 February, 127-132 and Adv. Materials, 16 (2004), 73-76. Peng, et al. have shown a way to make nano pillars on silicon by a chemical method. According to this method, a silicon wafer, which may be n- or p-type and has the {111} face exposed to solution, is etched at 50° C. using the following solution: 5M HF and 20 mM AgNO3. The mechanism postulated in these papers is that isolated nanoclusters of silver are electrolessly deposited on the silicon surface in an initial stage (nucleation). In a second (etching) stage, the silver nanoclusters and the areas of silicon surrounding them act as local electrodes that cause the electrolytic oxidation of the silicon in the areas surrounding the silver nanoclusters to form SiF6 cations, which diffuse away from the etching site to leave the silicon underlying the silver nanocluster in the form pillars.
K. Peng et al., Angew. Chem. Int. Ed., 44 (2005), 2737-2742; and K. Peng et al., Adv. Funct. Mater., 16 (2006), 387-394, relate to a method of etching a silicon wafer that is similar to that described in the earlier papers by Peng et al but the nucleation/silver nanoparticle deposition step and the etching step are performed in different solutions. In a first (nucleation) step, a silicon chip is placed in a solution of 4.6M HF and 0.01M AgNO3 for 1 minute. A second (etching) step is then performed in a different solution, namely 4.6M HF and 0.135M Fe(NO3)3 for 30 or 50 minutes. Both steps are carried out at 50° C. In these papers, a different mechanism is proposed for the etching step as compared to the earlier papers, namely that silicon underlying the silver (Ag) nanoparticles are removed and the nanoparticles gradually sink into the bulk silicon, leaving columns of silicon in the areas that are not directly underlying the silver nanoparticles.
In order to increase the uniformity and density of the pillars grown on silicon wafers and the speed of growth, it has been proposed in WO2007/083152 to conduct the process in the presence of an alcohol.
WO2009/010758 discloses the etching of silicon powder instead of wafers, in order to make silicon material for use in lithium ion batteries. The resulting etched particles, an example of which is shown in FIG. 2, contain pillars on their surface and the whole of the resulting particles can be used in the anode material of the batteries; alternatively, the pillars can be severed from the particles to form silicon fibres and only the silicon fibres are used to make the anode. The etching method used is the same as that disclosed in WO2007/083152.